Position sizing is the most important skill in trading. More important than entry signals. More important than identifying the trend. Yet most traders ignore it completely, randomly buying whatever quantity "feels right" on any given trade. This article teaches you how to calculate the right trade size every time, why it matters, and how different methods compare so you can choose what works for you.
Why Position Sizing Is the Most Important Skill in Trading
Here's the uncomfortable truth: your position size determines your success more than your entry signal does.
Imagine two traders. Both identify the exact same trade opportunity. Both buy at the same price, both place stops at the same level. The only difference: Trader A calculates position size using risk management (position sizes vary based on volatility). Trader B just buys the same number of shares in every trade, regardless of setup.
Over 100 trades, Trader A's results will be far superior, not because of better entries, but because their position sizes adapt to market conditions. On volatile days, they trade smaller. On calm days, they trade slightly larger. Over time, this adaptation compounds into massive differences in profit.
Here's another truth: you can have a mediocre trading system with perfect position sizing and still make a fortune. You can have an excellent trading system with bad position sizing and still go broke. Position sizing is that important.
The Fixed Fractional Method (Step By Step)
This is the most popular method among professional traders. It's simple, it's consistent, and it automatically adjusts position sizes as your account grows.
The Concept
You decide to risk a fixed fraction of your account on each trade (typically 1-2%). Then, you calculate how many shares you can buy while staying within that risk limit.
Step-by-Step Process
Step 1: Know your account size
Let's say you have £50,000.
Step 2: Decide your risk percentage
We'll use 1% (conservative approach). Some traders use 2%.
Step 3: Calculate your risk amount
Risk amount = Account size × Risk % = £50,000 × 1% = £500
So on this trade, you can lose a maximum of £500.
Step 4: Identify your stop loss level
Let's say you're trading HSBA. You want to buy at 820p with a stop loss at 800p.
Step 5: Calculate your risk per share
Risk per share = Entry price - Stop loss price
Risk per share = 820p - 800p = 20p = £0.20 per share
Step 6: Calculate position size
Position size = Risk amount ÷ Risk per share
Position size = £500 ÷ £0.20 = 2,500 shares
Step 7: Execute the trade
Buy 2,500 shares of HSBA at 820p. Your stop loss is 800p. Your maximum loss is 2,500 × £0.20 = £500 (exactly your 1% risk).
Real-World Examples
Example 1: More Volatile Stock
Account: £50,000, Risk 1% = £500.
Stock: Smaller cap FTSE 250 stock at 150p. Stop loss at 135p.
Risk per share: 150p - 135p = 15p = £0.15
Position size = £500 ÷ £0.15 = 3,333 shares
The stop loss is tighter (smaller distance in pence), so you buy more shares.
Example 2: Less Volatile Stock
Account: £50,000, Risk 1% = £500.
Stock: FTSE 100 large-cap at 3,000p. Stop loss at 2,850p.
Risk per share: 3,000p - 2,850p = 150p = £1.50
Position size = £500 ÷ £1.50 = 333 shares
The stop loss is wider, so you buy fewer shares.
How Position Size Adapts
Notice something important: in both cases, your risk is exactly £500. Your position size automatically adapted to the volatility of the stock. This is the genius of fixed fractional sizing. You don't have to decide "should I buy 1,000 shares or 5,000 shares." The maths decides for you.
Volatility-Based Sizing Using ATR
The fixed fractional method works well, but some traders prefer basing their stop loss distance on volatility explicitly, using the Average True Range (ATR) indicator.
The Concept
ATR measures how much a stock moves on average. High ATR = high volatility. Low ATR = low volatility. You use ATR to set your stop loss, then calculate position size from there.
Step-by-Step Process
Step 1-3: Same as fixed fractional
Account: £50,000. Risk 1% = £500.
Step 4: Check the ATR
Let's say the ATR is 15 points on HSBA.
Step 5: Set your stop loss using ATR multiple
You decide to use 1.5× ATR as your stop distance.
Stop distance = 1.5 × 15 = 22.5 points
Step 6: Calculate actual stop price
If you're buying at 820p, your stop is 820p - 22.5p = 797.5p
Step 7: Calculate position size
Risk per share = 22.5p = £0.225
Position size = £500 ÷ £0.225 = 2,222 shares
Why This Method Is Useful
On a highly volatile day, ATR is high, so your stop is wider, so your position is smaller. On a calm day, ATR is low, so your stop is tighter, so your position is larger. This adapts to real-time market conditions, not just historical volatility.
Some traders prefer this because it feels more dynamic. Others stick with fixed fractional because it's simpler. Both work.
The Kelly Criterion (Simplified)
The Kelly Criterion is a formula used in betting and investing to calculate the optimal position size based on your win rate and risk-reward ratio. It's mathematically elegant but requires knowing your historical performance.
The Formula
Kelly % = (Win% × AvgWin – Loss% × AvgLoss) ÷ AvgWin
Where:
- Win% = your win rate as a decimal (0.40 for 40%)
- AvgWin = your average profit per winning trade (in pounds)
- Loss% = your loss rate as a decimal (0.60 for 40% win rate)
- AvgLoss = your average loss per losing trade (in pounds)
Example Calculation
Let's say your trading history shows:
- Win rate: 55%
- Average win: £1,000
- Average loss: £800
Kelly % = (0.55 × £1,000 – 0.45 × £800) ÷ £1,000
Kelly % = (£550 – £360) ÷ £1,000
Kelly % = £190 ÷ £1,000
Kelly % = 0.19 = 19%
This suggests you should risk 19% of your account per trade for maximum growth.
Important: The Kelly Criterion Is Too Aggressive
The Kelly Criterion calculates maximum growth, but it also means massive drawdowns. A 19% risk per trade will see 50%+ drawdowns regularly. Most traders use a "half Kelly" or "quarter Kelly" approach for safety.
So you might risk only 9.5% (half Kelly) or even 4.75% (quarter Kelly) instead of the full 19%.
When to Use It
The Kelly Criterion is useful if you have documented trading results and you want to optimize your sizing based on your actual win rate and risk-reward. But it requires real data. If you're new to trading, stick with fixed fractional (1-2% risk).
Position Sizing Calculator Walkthrough
Most modern trading platforms have built-in position sizing calculators. Here's how to use one:
Typical Calculator Fields
- Account size: £50,000
- Risk percentage: 1%
- Entry price: 820p
- Stop loss price: 800p
Calculator outputs:
- Risk amount: £500
- Risk per unit: £0.20
- Position size: 2,500 shares
- Max loss: £500
Some advanced calculators also show:
- Price at which you'd lose your risk (the stop price)
- If you set a target, your expected profit and R:R ratio
Popular platforms offering position calculators: IG, Interactive Brokers (Trader Workstation), Think or Swim (TD Ameritrade in US), and many others.
How Position Size Changes With Account Growth
One of the beauties of percentage-based sizing: as your account grows, your position size automatically grows.
Year 1: Account £50,000
Trade setup: same as before (820p entry, 800p stop, 20p risk).
Position size = £500 (1%) ÷ £0.20 = 2,500 shares
Year 2: Account Now £75,000 (you've been profitable)
Same trade setup: 820p entry, 800p stop, 20p risk.
Risk amount = £75,000 × 1% = £750
Position size = £750 ÷ £0.20 = 3,750 shares
Without changing your system at all, you now trade 1,250 more shares because your account is larger.
Year 3: Account Now £150,000
Same trade setup again.
Risk amount = £150,000 × 1% = £1,500
Position size = £1,500 ÷ £0.20 = 7,500 shares
You're now trading 3 times the shares you started with, without changing your approach. This is compounding at work.
The Important Part
Notice that your risk amount doubled from Year 1 to Year 2 (£500 to £750 to £1,500), but your risk percentage stayed at 1%. This is the safety valve of percentage-based sizing. As you make money, you naturally trade bigger, but you never risk more than your predetermined percentage.
Scaling In and Scaling Out
So far we've discussed entering a full position at once. But many traders scale in (add to position over time) or scale out (exit position in pieces).
Scaling In
You don't buy your full 2,500 shares at entry (820p). Instead, you might buy 1,250 at 820p, then add 1,250 if price confirms your thesis by moving to 830p.
Pros: You're averaging in at better prices if the setup works. You catch the move earlier. Cons: If the trade fails immediately, you only caught half of it.
Best practice: If you scale in, your first entry should use your full position sizing maths. Your second (and any subsequent) entries should use a smaller amount, say 50% more, again calculated with 1% risk. This keeps you disciplined.
Scaling Out
Instead of selling all 2,500 shares at your 1:2 target (840p), you might sell 1,250 at 840p (locking in your 1:2), then hold 1,250 longer hoping for your 1:3 target at 860p.
Pros: You lock in profits early, then let winners run. Cons: More complex execution, you might miss the bigger move if you exit first half too early.
Best practice: Create a scaling plan before you enter. For example: "I'll sell 50% at 1:2 risk-reward, hold 25% for 1:3, and hold final 25% for 1:5."
Position Sizing for Different Instruments
The principles are the same across all instruments, but execution varies slightly:
UK Stocks (What We've Been Discussing)
You buy shares at pence prices. Position size calculation is straightforward. Use whole shares (you can't buy 0.5 shares on most UK brokers).
CFDs (Contracts for Difference)
CFDs are derivatives that track stock prices. Instead of owning shares, you're speculating on price direction. Position sizing is similar, but you specify contract sizes, not share quantities.
Example: A HSBA CFD contract might be worth £1 per point. If you want to risk £500 and your stop is 20 points away, you'd buy 25 contracts (£500 ÷ £20 per contract = 25).
Spread Betting
Similar to CFDs. You specify stake per point. If you stake £10 per point and you stop loss is 20 points away, your max loss is £200 per point × stake.
Example: To risk £500, you'd stake £25 per point (£500 ÷ 20 point stop = £25/point).
Index Futures
Futures contracts have fixed multipliers. FTSE 100 futures = £10 per index point. So if the FTSE is at 8,000 and you want to trade one contract with a 50-point stop, your risk is 50 points × £10 = £500.
Currencies (Forex)
Forex is traded in lots. Standard lot = 100,000 units. Mini lot = 10,000 units. Your position size is expressed in lots, and your risk is calculated based on the pip movement and lot size.
This is more complex and beyond the scope of UK stock trading, but the principle is identical: calculate your position size to match your risk percentage.
Common Position Sizing Mistakes
Mistake 1: Using the Same Size for Every Trade
You always buy 1,000 shares. No matter the stock, no matter the volatility, no matter the stop distance. This is how traders take oversized losses on volatile stocks and undersized wins on stable ones.
Fix: Use percentage-based sizing always. Calculate new size for every trade.
Mistake 2: Increasing Size After Losses
You take a loss. You're frustrated. The next trade, you "make an exception" and size up to 1.5x your normal size. That trade also loses. Now you've lost 1.5× your usual amount. This is revenge sizing and it destroys accounts.
Fix: Stick to your percentage, always. If anything, size down after losses until you regain confidence.
Mistake 3: Ignoring Your Portfolio Heat
You calculate each position size correctly in isolation. But you don't check your total portfolio heat. You end up with five trades on, each sized at 1% risk, meaning 5% total portfolio heat when you wanted 3%.
Fix: Always check total heat. If you're at max, don't open new positions.
Mistake 4: Not Adjusting for Market Conditions
During a market crisis with 10x normal volatility, you still use your usual position sizes. One gap move wipes you out. During the quietest market in years, you stick to conservative sizing when you could trade larger.
Fix: At minimum, use ATR-based sizing to adjust for volatility. Or at least manually review position sizes on extreme days.
Mistake 5: Rounding Incorrectly
Your maths says buy 2,487 shares. You round up to 2,500 because it's a nicer number. Now you're risking slightly more than 1%. Over many trades, these rounding errors accumulate.
Fix: Round down, never up. If your maths says 2,487, buy 2,400. Conservative is safer than aggressive.
Summary
Position sizing is more important than your entry signal. Use the fixed fractional method: risk a fixed percentage of your account (1-2%) on each trade, then calculate position size to match that risk. As your account grows, your position sizes automatically grow. If you're a more sophisticated trader, consider ATR-based sizing or Kelly Criterion. But for most traders, fixed fractional is perfect. Calculate a new position size for every trade. Never use the same size for every trade. Check your total portfolio heat before opening new positions. Position sizing seems mechanical and boring, but it's the difference between traders who survive and traders who blow up.